is there any way to use an iMac 27" mid 2011 as a display for a newer iMac or MacBook?

Welcome to Apple Support Communities.

Sorry, that's not possible.

Please refer to: Use your iMac as a display with target display mode - Apple Support as to why it won't work.

Jack

See https:///astropad.com/product/lunadisplay. Using Luna Display I have been able to use my 2013 iMac monitor with my 2020 Mac mini m1 but not without problems. I have finally given up on getting this configuration to consistently work correctly and purchased a separate monitor for the Mac mini. If anyone out there has been 100% successful with this configuration I'd sure like to hear how they did it.